The subject matter described herein is related in a general sense to that described in Beck, et al. U.S. Pat. No. 5,006,209 ('209) issued Apr. 9, 1991 and entitled "ELECTROLYTIC REDUCTION OF ALUMINA", and the disclosure thereof is incorporated herein by reference.
The invention embodied in the subject matter described herein was made during work financed by the following government contracts: NSF Phase I SBIR ISI 8851484; NSF Phase II SBIR ISI-8920676; and DOE Contract DE-FG01-89CE15433.
The present invention relates generally to the electrolytic reduction of alumina to aluminum and more particularly to an anode and to a lining for the cell used in the electrolytic reduction process.
The aforementioned Beck, et al. '209 patent is directed to a method and apparatus for the electrolytic reduction of alumina to aluminum. The electrolytic reduction is performed in an electrolytic reduction vessel having a plurality of vertically disposed, non-consumable anodes and a plurality of vertically disposed, dimensionally stable cathodes in closely spaced, alternating arrangement with the anodes. The vessel contains a molten electrolyte bath composed of (1) NaF+AlF.sub.3 eutectic, (2) KF+AlF.sub.3 eutectic and (3) LiF. In one embodiment, a horizontally disposed, gas bubble generator is located at the vessel bottom, underlying the cathodes and the spaces between each pair of adjacent electrodes.
Finely divided particles of alumina are introduced into the bath where they are maintained in suspension in the molten electrolyte by rising gas bubbles generated at the anodes and at the gas bubble generator, during the electrolytic reduction process. The horizontally disposed, gas bubble generator may be an auxiliary anode or anode part located at substantially the bottom of the electrolytic reduction vessel, in contact with the molten electrolyte bath, or it may be in the form of a gas sparger for bubbling air or nitrogen upwardly from the vessel bottom.
The molten electrolyte bath has a density less than the density of molten aluminum and less than the density of alumina. Metallic aluminum forms at each of the cathodes, during performance of the electrolytic reduction process, and the metallic aluminum flows downwardly as molten aluminum along each cathode toward the bottom of the vessel where the molten aluminum accumulates. The molten electrolyte bath is maintained at a relatively low temperature in the range of about 660.degree. C. to about 800.degree. C. (1220.degree.-1472.degree. F.). The molten electrolyte has a composition which provides a relatively low anode resistance, avoids excessive corrosion of the anode and avoids deposition of bath components on the cathodes.
The anodes disclosed in the aforementioned Beck, et al. '209 patent are composed of copper or of nickel ferrite-copper cermet. The electrolyte bath disclosed in the Beck, et al. '209 patent produced reduced corrosion on copper anodes, compared to the corrosion produced by other electrolyte bath compositions. However, the corrosion rate for the copper anodes was still subject to improvement.
Attempts have also been made to employ, as a non-consumable anode composition, a nickel ferrite-copper cermet In this connection, see U.S. Pat. Nos. 4,399,008 and 4,620,905, for example. However, a nickel ferrite-copper cermet anode has also proved to have significant drawbacks, and it has not proven to be feasible for the electrolytic reduction of alumina to aluminum on a commercial scale. U.S. Pat. No. 4,999,097 discloses an electrolyte cell for the electrolytic reduction of alumina to aluminum, and this cell employs an anode composed of a foundation metal which can be, among others, copper, nickel, steel or combinations thereof.
The cell employed in conventional processes for the electrolytic reduction of alumina to aluminum comprises a vessel for containing a molten electrolyte usually composed of halides. The vessel has an external shell and has an interior lined with various materials. The bottom of the vessel has a layer of refractory material, e.g. alumina, adjacent the external shell, and the interior is lined at the bottom with carbon or graphite blocks. The walls of the cell also are lined with carbon or graphite blocks, but unlike the bottom, the walls are not insulated with a refractory material.
The seams between the blocks are filled with carbon paste. During operation of the cell, the molten electrolyte penetrates into any unfilled seams or voids or cracks in the interior lining. Penetration of the electrolyte into the lining causes the lining to deteriorate. Penetration occurs up to a level called the freeze line, which is the level on the uninsulated walls where enough heat is lost from the molten electrolyte to cause it to freeze. Generally, there is a frozen ledge at this level and above, composed of solidified electrolyte and alumina.
After 1,000 to 3,000 hours of operation, the interior lining of the vessel deteriorates to the point where it must be replaced. Disposal of spent lining removed from the vessel is a problem, with piles of spent lining accumulating around aluminum reduction plants.
In a cell of the type disclosed in the aforementioned Beck, et al. '209 patent, an excess of alumina is introduced into the molten electrolyte, and the resulting bath composition allows the use of alumina refractory brick to line the interior walls of the vessel. Because the walls are thus thermally insulated, the frozen ledge is eliminated, which is desirable. However, the alumina bricks which line the walls on the interior of the vessel are subject to the same penetration problems as carbon blocks, even though the alumina blocks will last longer.
It would be desirable to have an interior lining for the vessel which is not subject to electrolyte penetration, which is easy to replace, which can be readily recycled and which allows the entire vessel to be thermally insulated.